That's not at all how a matchamaker works...
The more you win, the more the matchmaker increases its estimation of your skills. For every game, it then tries to match you with other people from the same skill level. That means, after a winning streak, better people against you AND better people with you, not worse, that's just not true.
Once the matchmaker has precisely determined your skill level and the skill level from everyone else, you should, statistically, win 50% of your games, and that's why we say that the matchmaker tries to make sure everyone is at 50% winrate. But the matchmaker never purposefully gives you matches impossible to win. Especially since it means it would give the same impossible-to-win game to the 4 other people on your team who are NOT on a winning streak, so it can't even be consistent.
What you observe is due to the fact that it's pretty hard to have a precise estimation of someone skill level in a 5v5 game, so it can take quite a large number of games before getting a good estimation, plus people are not always consistent over time and can improve as time passes by, and so the matchmaking process is full of error, but it doesn't work at all how you say it does.